This Error event indicates that Microsoft Office Outlook
Web App did not initialize because the Outlook Web App base themes folder
referenced in the event description does not exist. The base folder contains
images and cascading style sheets that are used to control the default appearance
of Outlook Web App for your organization. The base folder is also used by other
custom Outlook Web App themes if they are installed. By default, the base
folder is located in D:\Program Files\Microsoft\Exchange
Server\ClientAccess\Owa\<version>\themes\.

Note: - If you installed Exchange 2010 SP2 or SP3 you will see
two different versions (14.3.158.1 & 14.3.174.1) of folders.

Resolution: -

To resolve this issue, we need to replace the Theme files from
the Exchange Installation DVD, however if you have any other working servers
than we can copy the existing files from and replace it on the problematic
server.

1.Stop the IIS
Admin Service.

2.Stop Word Wild
Web service.

3.Copy the Themes
folder from Exchange Installation DVD/ Working server and Replace on
Problematic Server.

4.Start IIS Admin
Service.

5.Start Word Wild
Web service.

Now try to access the Webmail it will work like normal and we
can confirm on the Event ID.